This invention relates, in general, to a novel process for the preparation of an organic pigment and, in particular, to a process for the preparation of alpha metal-free phthalocyanine. More specifically, the invention relates to a process for the preparation of alpha phthalocyanine pigment particles ranging in size from about 1 micron to about 75 microns.
Phthalocyanine, which is also known as tetrabenzotetraazaporphin and tetrabenzoporphyrazine, may be said to be the condensation product of four isoindole groups. Metal-free phthalocyanine has the following structure: ##SPC1##
In addition to the metal-free phthalocyanine of the above structure, various metal derivates of phthalocyanine are known in which the two hydrogen atoms in the center of the molecule are replaced by metals from any group of the periodic table. Also, it is well known that from one to 16 of the peripheral hydrogen atoms in the four benzene rings of the phthalocyanine molecule may be replaced by halogen atoms and by numerous organic and inorganic groups.
Metal-free phthalocyanine is known to exist in at least three polymorphic forms, including alpha, beta, and "x-form". X-form is a newly discovered polymorphic form of phthalocyanine which has been shown to have unique xerographic properties. X-form is further discussed in a copending application, Ser. No. 375,191, filed in the United States Patent Office on June 15, 1964. The various forms of metal-free phthalocyanine may easily be distinguished by comparison of their X-ray diffraction patterns and/or infrared spectra. (See copending application, Ser. No. 505,723, filed in the United States Patent Office on Oct. 29, 1965.)
The alpha form of phthalocyanine has several important uses. For example, it has been found to be commercially useful in plastic, ink, and paint applications. Further, it is known that alpha phthalocyanine may be used alone or in conjunction with X-form phthalocyanine as a photoconductive material in xerography. In addition, it is known that the alpha form of phthalocyanine may be converted to the X-form by any suitable grinding process, such as ball milling or salt milling. Moreover, alpha phthalocyanines have found significant use in manifold imaging described in copending application Ser. No. 708,340, filed in the U.S. Patent Office on Feb. 26, 1968 and in particle migration imaging systems such as photoelectrophoresis described in U.S. Pat. No. 3,384,448 and copending application Ser. No. 560,603, filed in the U.S. Patent Office on June 27, 1966.
In a photoelectrophoretic imaging system, various colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be intensely colored and electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating radiation, through interaction with one of the electrodes. The images are produced in color because mixtures of two or more differently colored sets of particles which are each sensitive only to light of a specific wavelength or narrow range of wavelengths are used. Particles used in this system must have both intense pure colors and be highly photosensitive.
Copending application Ser. No. 708,380 describes an imaging system utilizing a manifold sandwich comprising an electrically photosensitive material between a pair of sheets. In this imaging system, an imaging layer is prepared by coating a layer of electrically photosensitive imaging material onto a substrate. In one form the imaging layer comprises a photosensitive material such as alpha phthalocyanine dispersed in a cohesively weak insulating binder. This coated substrate is called the donor. When needed, the imaging layer is rendered cohesively weak. The process step of weakening the imaging layer is termed activation and in most cases the imaging layer is activated by contacting it with a swelling agent, solvent, or partial solvent for the imaging layer or by heating. This step may be eliminated, of course, if the layer retains sufficient residual solvent after having been coated on the substrate from a solution or paste or is sufficiently cohesively weak to fracture in response to electromagnetic radiation, normally visible light and electrical field. After activation a receiver sheet is laid over the surface of the imaging layer. An electrical field is then applied across this manifold sandwich while it is exposed to a pattern of light and shadow representative of the image to be reproduced. Upon separation of the donor substrate or sheet and receiver sheet the imaging layer fractures along the lines defined by the pattern of light and shadow to which the imaging layer has been exposed. Part of the imaging layer is transferred to one of the sheets while the remainder is retained on the other sheet so that a positive image, that is, a duplicate of the original is produced on one sheet while a negative image is produced on the other.
In the preparation of alpha phthalocyanine for use in the above-mentioned imaging systems, many difficulties have been encountered. The use of alpha phthalocyanine in particle migration systems places certain requirements on the crystal size of this material. For example, it is preferred that the alpha phthalocyanine crystals be substantially pure and of about 1 micron to about 75 microns in size. It appears that when substantially hexagonal crystals of this size are employed, color separation takes place more easily and is more complete.
The preparation of alpha phthalocyanine by previously known techniques, such as those disclosed in "Phthalocyanine Compounds" by Moser and Thomas, Rheinhold Publishing Company, pages 104-189, generate crystals of submicron size. Moreover, the use of most organic solvents for crystal growing, such as, for example, o-dichlorobenzene or dimethyl formamide, generate large crystals of the beta polymorph. While it is true that beta phthalocyanine may be converted to the alpha form by dissolving it in 98% sulfuric acid solution and precipitating the solution in ice water, sulfuric acid tends to degrade the phthalocyanine resulting in the formation of phthalimide, phthalic acid, and various nitrogen containing compounds which are undesirable in a particle migration system. Moreover, the alpha phthalocyanine which is generated is in the form of submicron particles.
It is, therefore an object of this invention to provide alpha phthalocyanine material devoid of the above-noted disadvantages.
It is another object of this invention to provide a method for the preparation of alpha phthalocyanine crystals for use in particle migration imaging systems.
Another object of this invention is to generate substantially hexagonal alpha phthalocyanine particles ranging in size from about 1 micron to about 75 microns.
Still another object of this invention is to provide a method of synthesis of alpha phthalocyanine in which the final produce is free of sulfuric acid contamination and decomposition products.
A further object of this invention is to provide improved photoelectrophoritic imaging systems.
Yet another object of this invention to provide improved manifold imagining systems.
It is still another further object of this invention to provide improved color separation in particle migration imaging systems.